Veterinary Epidemiology DrDomingo

Reviewer for Vet. Epidem. (Draft copy prepared by Dr. R. D. Domingo; [email protected]); Version 0512 Page 1

Veterinary Epidemiology

Reviewer


Table of Contents

Introduction to Veterinary Epidemiology ................................................................................................... 3

Disease Determinants ......................................................................................................................................... 6

Transmission and Natural History of Disease ........................................................................................ 10

Measurement of Disease and Production ................................................................................................. 13

Disease Patterns ................................................................................................................................................. 19

Epidemiological studies ................................................................................................................................... 21

Sampling ................................................................................................................................................................ 24

Veterinary Diagnostics ..................................................................................................................................... 26

Association and causation .............................................................................................................................. 29

Outbreak Investigation .................................................................................................................................... 32

Surveillance and Monitoring ......................................................................................................................... 33

Disease Control ................................................................................................................................................... 36


Introduction to Veterinary Epidemiology

Definitions Epidemiology from Greek words: epi, meaning on or upon demos, meaning people logos, meaning the study of epi + demos + logos= the study of disease in a population “Epidemiology is the study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to the control of health problems” (Last, 1995).

Key words in understanding epidemiology Study- Epidemiology is a systematic and ordered approach to solve health or productivity

problems. Disease- Epidemiology seeks to understand disease or productivity issues affecting the host

population in order to create appropriate control measures. Distribution- the disease picture is clearly described according to animal, time and place Determinants -refer to factors or events that are capable of bringing about a change in

health. Populations- Epidemiology deals with groups of people rather than with individual

patients. Control- epidemiology aids in developing and evaluating interventions to control and

prevent health problems.

Historical highlights 3500B.C. Biblical guidelines on disease control and prevention

Sanitation. Wash when exposed. Dispose waste. Quarantine the sick (lepers) Disinfection. Hyssop- the first antiseptic.

460-377 B.C. Hippocrates - as the Father of medicine; He described the distribution of diseases by season, age, climate and personal behavior; He introduced the word epidemeion to refer to diseases that affect the community 1700’s James Lind – Experimental Epidemiology on scurvy 1749-1823 Edward Jenner –Pioneer in Disease control through vaccination (using cowpox against smallpox) 1818-1865 Ignác Fülöp Semmelweis – “Father of infection control” and the "Savior of mothers." Instituted hand washing among medical personnel 1813-1858 John Snow – “Father of epidemiology”; In 1832 cholera struck London, Snow mapped the cases, identified the water source and removed the water pump handle 1807–1883 William Farr - The father of modern vital records 1910 Florence Nightingale- Developed a Model Hospital Statistical Form for hospitals


1822–1895 Louis Pasteur- his experiments supported the germ theory of disease- that microorganisms are the true cause of many diseases; Production of the first vaccine against anthrax, rabies 1900- 1938 Role of carriers in transmission: Typhoid Mary & George Soper. Soper discovered Mary Mallon, an immigrant from Ireland, as the typhoid carrier. 1946 Establishment of CDC 1947 The Framingham Study- the prototype and model of the cohort study; Research on cardiovascular disease; For the past 50 years, the study did interviews, physical exams, laboratory tests, and other tests every 2 years. 1970’s Invention and continuing evolution of microcomputer technologies 1980 CDC develops software packages for epidemiology; Example: Epi Info 1990’s Development and application of techniques in molecular biology

Objectives of epidemiology 1. CONTROL. To plan, monitor and evaluate disease control programs 2. DETERMINANTS. To determine the agent, host, and environmental factors that

affect health or disease occurrence 3. COST. To assess the economic effects of a disease and analysis of the costs and

economic benefits of proposed disease control programs 4. PREDICT. To predict the occurrence of disease. (Friis & Sellers, 2009) 5. RANK. To establish priorities for research and action 6. Objectives of epidemiology 7. OVERVIEW. To describe the health status of populations 8. POPULATION AT RISK 9. OUTBREAK investigation 10. SURVEILLANCE 11. ECOLOGY

Approaches (Types) of Epidemiological Investigations 1. Descriptive Epidemiology 2. Analytical epidemiology 3. Theoretical Epidemiology 4. Experimental Epidemiology


Comparison of Epidemiology with other disciplines

Clinician Pathologist Epidemiologist Patient Sick Individual Dead individual Population Usual Setting Hospital or clinic Laboratory “Field” Primary objective Treat the individual Pinpoint

abnormal forms or functions

Control the disease

Diagnostic procedure

Clinical (organoleptic) examination

Utilization of laboratory tools or reagents

Analysis of disease patterns or frequencies

Questions asked What is it? How do I treat it?

What’s abnormal with the sample? What’s the possible pathogenesis?

How is the disease distributed in terms of time, animal and place? What are the disease determinants? How can this disease be controlled effectively?

Modified from Schwabe, Riemann, & Franti (1977).


Disease Determinants

Definition: A determinant is any factor or variable that can affect the frequency with which a

disease occurs in a population. Synonym: Risk factor

Types of Determinants A distal determinant is one that is remote, either in position, time or resemblance to the outcome of concern. A proximal determinant is one that is much closer to the outcome of concern

Classification of determinants 1. According to degree of effects

Primary- factors whose variation exerts a major effect in disease causation. Secondary- predisposing, enabling or reinforcing factors.

2. According to location Intrinsic (endogenous) - refers to physical or physiological characteristics of the host or disease agent or intermediate host/vector (ex. genetic, age, physiological state). Extrinsic (exogenous) - related to environmental effect (e.g., climate, soil)

3. According to the epidemiologic triad

Host Factors The host refers to the animal or the person that is susceptible to the effect of the agent.

Types of hosts

1. Definitive (primary) host - One in which the parasite reaches sexual maturity. 2. Intermediate or secondary host - One in which the parasite does not reach sexual

maturity; the parasite is in a larval or asexual state. 3. Transport or paratenic host- the host that serves purely for mechanical transporting

of agents; no development of the organism occurs. 4. Incidental or tangential host- One which is not necessary for the perpetuation of

the parasite but may be involved in a disease process. 5. Reservoir host- The host species in which the parasite is usually maintained over

long periods of time.


Examples of host-related disease determinants

Agent May be defined as an element, a substance or a force, either animate or inanimate, the presence or absence of which, may following effective contact with susceptible animal/human host under proper environmental conditions, serve as stimulus to initiate or perpetuate a disease process

Categories

Agent Determinants

1. Infectivity -The capability to enter, survive, and multiply in the host. 2. Pathogenicity -The power of an organism to produce disease. 3. Pathogenicity – The ability to cause disease 4. Virulence - The degree of pathogenicity; The power to cause deaths 5. Immunogenicity- The ability of an infectious agent to induce specific immunity. 6. Antigenic variation - The ability to mutate frequently gives certain pathogens the

high potential to cause outbreaks.


7. Infectious Dose- It refers to the minimum amount of pathogen (measured in number of microorganisms) required to cause an infection in the host.

8. Gradient of infection- The response of an animal to challenge by an infectious agent a. Colonization (S. aureus in skin and normal nasopharynx) b. Subclinical or inapparent infection (polio) c. Latent infection (virus of herpes simplex) d. Manifest or clinical infection

9. Outcome of Infection a. Types of carrier state according to disease state:

i. True carrier= Healthy or Asymptomatic ii. Incubatory carrier= or prodromal

iii. Convalescent carrier b. Types of carrier state according to shedding duration:

i. Temporary carrier ii. Chronic carrier

10. Host Range- Range of hosts an agent can infect. The wider is the range of hosts, the greater is the probability of the infectious agent to survive, persist and cause disease.

11. Reservoir- It is the natural habitat of the infectious agent. This habitat can be any person, animal, arthropod, plant, soil, or substance, or a combination of these.

Source: Lt Col A. S. Kushwaha. 2006. Lecture notes in Infectious Disease Epidemiology


ENVIRONMENT (Extrinsic) The environment influences the existence of the disease agent. It affects the host’s exposure or susceptibility to an etiologic agent.

Classification

Additional notes 1. Climate- Climate or macroclimate refers to the weather situation over a long period of time, usually 30 years or more.

Microclimate - refers to the actual climatic conditions covering the specific, limited environment where the host, agent, and vector or intermediate host exist in dynamic interaction.

Consists of rainfall, temperature, solar radiation, humidity and wind Weather refers to the conditions of a certain moment as opposed to climate, the

long-term average of weather conditions. 2. Soils

Direct effect on disease: Reservoir for certain pathogens Indirect effects on disease: Amount of vegetation= nutrition to ruminants; Minerals

supplied to the vegetation= mineral imbalances in animals 3. Man

Manipulates the environment where the animals are kept. He decides on the housing, waste drainage, water system, irrigation, etc.

Imposes certain management intervention directly affecting the animals. Examples – chemotherapy, vaccination, quarantines, movement control, etc.


Transmission and Natural History of Disease

Natural history of Disease The disease timeline that covers the progression of a disease process in an individual over time, in the absence of treatment, from inception to resolution. Syn= Disease ecology

Stages 1. Stage of Exposure 2. Stage of Incubation 3. Stage of clinical manifestation 4. Stage of disease outcome.

Symptoms vs signs Symptoms are the complaints the patient can tell you about (e.g. headache, vomiting, dizziness). Signs are the features that would only be detected by a trained health worker (e.g. high temperature, fast pulse rate, enlargement of organs in the abdomen).

Stage of disease outcome. 1. Recovery 2. Chronic infection 3. Latent infection 4. Disability or defect 5. Death

Transmission

Reservoir 1. Human reservoirs- cases and carriers 2. Animal reservoirs- cases and carriers 3. Environment reservoirs- plants, soil and water


Portal of Exit 1. Gastrointestinal tract 2. Respiratory tract 3. Genitourinary tract 4. Skin and mucous membranes 5. Blood 6. Transplacental

Mode of Transmission The route of infection refers to the path which the disease agent used to gain access

to the body of the susceptible host Mode of transmission to the method of transference.

Mode and route

Fomites- Contaminated nonliving objects Vector-borne transmission

Biological transmission - pathogen replicates or develops in the vector Mechanical transmission- simple carriage of pathogen; no development or

multiplication Three types of biological transmission occur:

1. Developmental transmission: with an essential phase of development occurring in the vector 2. Propagative transmission: when the agent multiplies in the vector 3. Cyclopropagative transmission: a combination of 1 and 2

Portal of Entry and Exit The path by which an infectious agent enters a susceptible host. In most cases, same as portals of exit

Susceptible Host Host that is at risk for developing the disease

Herd Immunity Herd immunity refers to the resistance of a group to invasion and spread of an infectious agent, based on the resistance to infection of a high proportion of individual members of the group.


Sources of immunity

Immunity

Innate (Inborn; genetic

factors) Acquired

Species-determined inherent resistance to a disease agent, e.g., resistance of

humans to the virus of canine distemper.

Active (Produced own antibodies)

Passive (Antibodies from outside sources)

Natural Artificial Natural Artificial Through

exposure to infectious agent

Through intentional vaccination

Maternal antibodies

(usually transferred through the

milk)

Antibodies collected from

other immunized

animals


Measurement of Disease and Production

Population vs Population at risk Population- the whole collection of units (the “universe”) from which a sample may

be drawn Population at risk - the animals that are really susceptible to the disease being

studied.

Counts, ratio, proportion and rates 1. Count- This is a simple enumeration of the absolute number of cases of disease or

number of animals affected with a condition in a given population. Example: there are 40 carabaos diagnosed with trypanosomiasis in barangay Rizal last month.

2. Ratio- Ratio is the result of dividing one quantity by another Ratio= /𝑏 Examples Sex ratio= 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑓𝑒𝑚𝑎𝑙𝑒𝑠/𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑚𝑎𝑙𝑒𝑠

3. Proportion- A proportion is a special type of ratio, in which a is part of the denominator (a + b), is also called a proportion (as in the previous (female/all) example).

Proportion=

4. Rate- Rate is another type of ratio; Rates have the added dimension of time; Rates

measure the occurrence of an event in a population over time.

Rate =

x 10ⁿ


Measures of Morbidity

Prevalence Incidence

Period prevalence Point prevalence

Incidence Count Incidence risk or cumulative incidence Incidence density (also called true incidence rate) Attack rate (during a disease outbreak)

Incidence Measures the rapidity with which new cases are occurring in a population (how

quickly animals are catching the disease); Number of new cases in a given period in a specified population

Incidence count

The simple count of the number of cases of disease observed in a population. When used, it refers to the frequency of a disease in a population in which the

disease did not previously exist. Incidence counts are rarely used

Incidence Risk

Synonym: cumulative incidence (CI) An incidence risk is the probability that an animal will contract or develop a disease

in a defined time period. Because risk is a probability, it is dimensionless (that is, it has no units). The value

can be anywhere between O to 1. Incidence risk=

x 10 n

Incidence Rate

Synonym: incidence density or ID or incidence density rate Also called true incidence rate An incidence rate is the number of new cases of disease in a population per unit of

animal-time during a given time period. A measure of the average speed (velocity) at which the disease is spreading.

ID =


Attack rate

The term “attack rate” is often used instead of incidence during a disease outbreak in a narrowly-defined population over a short period of time.

Attack Rate =

X 100

Prevalence It describes the proportion of the population that is in the disease state at a specific

time. A snapshot of the situation at a single point in time. The term ‘prevalence rate’ is not a true rate because a rate should include units of

time.

Prevalence:

the number of cases of a given disease or attribute that exists in a population at a specified time. Example: 15 animals with mastitis

Prevalence risk:

the proportion of cases that has a specific disease or attribute at a specified point in time. Example: 15/200 animals with mastitis or 7.5 %.

Point Prevalence

It is the probability that a randomly selected animal suffers from that disease at a certain moment.

Period Prevalence

Period prevalence refers to the proportion of the population that had the disease during a specified period of time. Example: FMD period prevalence in 2009. It combines the point prevalence at the beginning of the period and the incidence (number of new cases that occur during the period).


Hypothetical Study

Modified from Stevenson (2005)

Number present at start

10 Number of withdrawn animals

2

Number present at end of study

8 Number of disease events

4

Prevalence in June AD and F or 3/9 33% Prevalence in December ADF and G or 4/8 50% Cumulative incidence

40% (4 cases in 10 animals)

Incidence density (exact)

4 cases per 80 cow-months at risk

Factors influencing observed prevalence rate

Source: Beaglehole, Bonnita, & Kjellstrom (1993)


Relationship between prevalence, incidence and duration of disease state

P ≈ IR × D Where: ≈ means approximately equal to. P = prevalence IR= incidence rate D = average duration of disease Interpretation High prevalence of a disease within a population might reflect high incidence or prolonged survival without cure or both. Conversely, low prevalence might indicate low incidence, a rapidly fatal process, or rapid recovery.

Measures of Mortality

Crude death rate

The crude mortality rate is the mortality rate from all causes of death for a population.

Crude mortality rate =

Case-fatality rate

It is actually a 'risk' measure (i.e. a proportion) instead of a 'rate.' Case fatality risk (or rate) refers to the incidence of death (proportion) among individuals who develop a specific disease (within a specified time period). Stated in another way, case fatality risk reflects the prognosis of disease among cases

Case fatality rate =

Cause-specific mortality rate

The cause-specific mortality rate is the mortality rate concerning a specified cause for a population during a specified time period. The numerator is the number of fatalities attributed to a specific cause while the denominator consists of the population at the midpoint of the time period. The resulting fraction is commonly converted to counts per 100,000 population.

Proportional mortality

These rates are used when the appropriate denominator is unknown and they are calculated by dividing the number of cases (or deaths) due to a specific disease by the number of cases (or deaths) from all disease s diagnosed.


Proportional morbidity/ mortality rates are often used for diagnostic laboratory data and are subject to variation in the numerator or the denominator. Hence, they are less preferable than measures of risk. As its name implies, proportional mortality is the proportion of all deaths that are due to a particular cause for a specifed population and time period:

Proportional mortality =

Measures of mortality illustrated

Crude death rate =

Where C represents the total number of animals in the study population, both sick and

healthy.

Case fatality rate =

Cause specific mortality rate =

Proportional mortality rate =


Disease Patterns

Disease patterns are described based on three main epidemiologic variables: Animal Time (temporal pattern) Place (Spatial pattern)

Animal Which type of animals are prone to develop the disease and which type tends to be spared ?„

Time How does disease frequency change over time, and what other factors are associated with these changes?

Classification of time trends 1. Short term 2. Cyclical 3. Seasonal 4. Long-term

Short term disease patterns

1. Sporadic disease: one that occurs only occasionally, i.e., generally absent, and unpredictable

2. Endemic disease: When diseases occur at a predictable frequency with slight deviations only.

Subclasses of endemic diseases a. Holendemic- expresses a high level of infection beginning early in life and

affecting most of the child population, leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (e.g. malaria).

b. Hyperendemic= high proportion of the population affected (the disease is constantly present at high incidence and/or prevalence rate and affects all age groups equally).

c. Mesoendemic = moderate proportion of the population affected d. Hypoendemic = small proportion of the population affected

3. Epidemic disease occurs above expected frequency in a population or given area over a particular period of time, i.e., disease events cluster in both place and time.

4. Pandemic- When epidemics occur at several continents – global epidemic


Epidemic curve The representation of the number of new cases of a disease by a graph, with the number of new cases on the vertical axis and calendar time on the horizontal axis, is the most common means of expressing disease occurrence

Classification of epidemic curves

1. Common source epidemic- A group of animals is exposed to a common disease-causing factor

a. Point source b. Intermittent c. Continuous

2. Propagated epidemic- Outbreak spreads gradually from animal to animal. 3. Mixed epidemic- A common source outbreak followed by secondary person-to-

person spread.

Cyclical Trend These are periodic peaks in disease frequencies occurring every 3 - 5 years.

Seasonal A special case of a cyclical trend where the observed fluctuations in disease incidence are related to seasonal changes.

Long-term or Secular trend The annual cases of a disease are plotted over a period of years Displays slow, regular long-term changes in disease frequency over long periods of time

Place

Where is the disease especially common or rare, and what is different about these places?

Types of Thematic Maps

a. Qualitative maps= maps that show non-measurable characteristics (e.g. Low and high rainfall).

b. Quantitative maps= maps that depict areas with measured variations

Types of quantitative maps:

(a) Dot maps (b) Choropleth maps (c) Isopleth maps (d) Proportional symbol maps


Epidemiological studies

Observational studies Subjects “merely observed” Classified into Descriptive studies and analytical studies

Some differences between descriptive and analytic studies


Cross-sectional study

Syn: disease frequency survey, prevalence study

A study that examines the relationship between diseases (or other health-related characteristics) and other variables of interest as they exist in a defi ned population at one particular time (Porta, 2008).

Analytical studies 1. Case control studies- To find out whether a risk factor is more common in animals

with a given disease (cases) than animals without the disease (controls). Often termed retrospective studies.

Clipart Source: http://www.myoops.org/twocw/tufts/courses/1/content/193106.htm

2. Cohort studies- A cohort (group) of healthy animals capable of developing the

disease of interest is divided into two groups. Differ only with regard to a specific risk factor. ollowed (monitored) forward for a period of time. Are often termed prospective studies.

Clipart Source: http://www.myoops.org/twocw/tufts/courses/1/content/193106.htm


Experimental Syn- intervention studies The researcher deliberately applies a preventive measure, such as

Clinical Trials

Any planned experiment which involves human or animal subjects and is designed to assess the effectiveness of one or more new treatments for a disease or preventive measures such as vaccines. Done in well-regulated conditions

Field trial

A comparative study involving new treatments or preventive measures applied under natural, field or semi-field conditions.


Sampling

Two types of cross-sectional study are commonly performed. Censuses Sample surveys or “Surveys

Definitions

Sample - A sample is a part of the population, selected by the investigator to gather information (measures) on certain characteristics of the original population

Sampling- the process of selecting a small number of units from a larger defined target group of units such that the information generated from the small group will allow inferences to be made about the larger group.

Target population - This is any complete, or the theoretically specified aggregation of study elements. It is usually the ideal population or universe to which research results are to be generalized.

Study population- The study population is the population to which the results of the study will be inferred.

Sampling frame- A list of sampling units from which units to be sampled can be selected. In most situations, it is difficult to get an accurate list.

Sampling unit - Basic sampling unit, BSU- the units which are chosen in selecting the sample.

Sampling scheme/method - Method used to select sampling units from the sampling frame. Sample size- the number of elements in the obtained sample Inference- the process of assuming that the disease status of the population is similar to the

disease status of the sample. Sampling error- the difference between the value of the parameter being investigated and

the estimates of this value based on the different samples.

General Categories of Sampling Methods 1. Probability sampling- Every unit in the population has a known probability of being

selected. The rules and procedures for selecting the sample and estimating the parameters are clearly defined.

2. Non-probability sampling- Probability of being selected is unknown

Types of Nonprobability Sampling Methods 1. Convenience sampling: relies upon convenience and access. 2. Judgment sampling: relies upon belief that selected sampling units fit characteristics 3. Quota sampling: emphasizes representation of specific characteristics. The first

stage consists of developing control categories, or quotas, of population elements. In the second stage, sample elements are selected based on convenience or judgment.

4. Snowball sampling: relies upon respondent referrals of others with like characteristics


Types of Probability Sampling Methods 1. Simple random sampling is a method of probability sampling in which every unit

has an equal nonzero chance of being selected 2. Systematic random sampling is a method of probability sampling in which the

defined target population is ordered and the sample is selected according to position using a skip or sampling interval.

3. Stratified random sampling is a method of probability sampling in which the population is divided into different subgroups (strata) and samples are selected randomly from each stratum.

4. Cluster sampling is a method of probability sampling in which the population is divided into a large number of groups, called clusters. Then a random sample of clusters is selected, based on a probability sampling technique such as SRS. Every element found in each cluster selected may or may not be included in the study.

5. Multi-stage sampling is a method of probability sampling wherein sampling a population is undertaken in different stages, with the sample unit being different at each stage

Problems with Survey Estimates The estimate from a survey is never exactly identical to the actual value in the population, even if all the procedures are done correctly.

1. Bias - Something is wrong with the way the sampling was done or the measurements taken.

a. Measurement bias- Caused by incorrect measurements b. Sampling bias- Caused by selection of a non-representative sample

2. Sampling error - Just by chance, even in the perfect survey, a sample selected randomly from a population will almost never be exactly the same as the entire population.


Veterinary Diagnostics

Definition “A test is any device or process designed to detect, or quantify a sign, substance,

tissue change, or body response in an animal.

Categories Screening Tests- Tests used for clinically healthy animals Diagnostic Tests- Tests for clinically diseased animals.

Qualities of a TEST Precision - replication of test results when the same samples are re-tested.

Synonyms: Reproducibility, reliability (repeatability) Validity- ability to distinguish between who has a disease and who does not.

Synonym: accuracy

Validity is expressed in four dimensions 1. Sensitivity 2. Specificity 3. Positive predictive value 4. Negative predictive value

Data Lay-out for Test Evaluation

True Disease Status (as measured by the Gold standard)

Row marginal

total Diseased Non-diseased

Diagnostic Test

Test Positive a (True Positive)

b (False positive)

a + b

Test Negative c (False negative)

d (True negative)

c + d

Column marginal total a + c b + d a + b + c + d

Gold Standard An accepted test that is assumed to be able to determine the true disease state of a patient regardless of positive or negative test findings or sensitivities or specificities of other diagnostic tests used


Sensitivity

The sensitivity of a test is the probability of the test to generate positive results among animals that actually possess the disease.

Sensitivity=

Specificity The specificity of the test is the probability of a test to generate negative results

among animals that are genuinely free of the disease.

Specificity =

Relationship between sensitivity and specificity

Positive predictive value It refers to the proportion of animals actually with the disease among all of the animals with positive test results. It answers the question: “If the test result is positive what is the probability that the

animal actually has the disease?"

Positive predictive value =

Negative predictive value It refers to the proportion of animals free of the disease among all of the animals with negative test results. It answers the question: “If the test result is negative what is the probability that the

animal does not have disease?"

Negative predictive value =


Apparent vs True Prevalence Apparent prevalence- The proportion of all animals that give a positive test result. Formula

Apparent Prevalence=

True Prevalence=

If sensitivity and specificity of a test are known, then the true prevalence and other values can be calculated

True prevalence=

Accuracy Accuracy is the proportion of all tests, both positive and negative, that are correct.

Use of Multiple Tests 1. Parallel – the tests are performed at the same time and interpreted together.

Test A Test B Diagnosis

(-) (-) Negative (+) (-) Positive (-) (+) Positive

2. Serial – the tests are performed sequentially. The results of the first test usually

determine whether the second test is still necessary or not. Only the positive cases are retested.

Test A Test B Diagnosis

(+) (-) Negative (-) (+) Negative (+) (+) Positive


Association and causation

Definitions Association is the degree of dependence or independence between two variables. An

association is present if probability of occurrence of a variable depends upon one or more variable.

Causality - The relating of causes to the effects they produce. The property of being causal. The presence of cause. Ideas about the nature of the relations of cause and effect. The potential for changing an outcome (the effect) by changing an antecedent (the cause). Porta, 2008

Types of association 1. Positive/Negative 2. Direct/Indirect 3. Causal/Non causal

Causes of disease A cause of a disease is an event, condition, or characteristic which plays an essential role in producing an occurrence of the disease.

Types of causes Necessary cause: must be present for a disease to occur. It must always precede an

effect. This effect need not be the sole result of the one cause. Sufficient cause: a set of minimal conditions and events inevitably producing disease

where all exposed individuals inevitably become cases.

Confounding variable or confounder Confounding= Latin: confundere = to mix together The effect of an extraneous variable that can wholly or partly account for an

apparent association between variables. Confounding can produce a spurious association between study variables, or can

mask a real association.

Concept of causation

Koch's postulates

In 1890 the German physician and bacteriologist Robert Koch set out his celebrated criteria for judging whether a given bacteria is the cause of a given disease. Koch's criteria brought some much-needed scientific clarity to what was then a very confused field.


Koch's postulates are as follows:

1. The bacteria must be present in every case of the disease. 2. The bacteria must be isolated from the host with the disease and grown in pure

culture. 3. The specific disease must be reproduced when a pure culture of the bacteria is

inoculated into a healthy susceptible host. 4. The bacteria must be recoverable from the experimentally infected host.

However, Koch's postulates have their limitations and so may not always be the last word. They may not hold if:

The particular bacteria (such as the one that causes leprosy) cannot be "grown in pure culture" in the laboratory.

There is no animal model of infection with that particular bacteria.

Bradford Hill’s Criteria

Sir Bradford Hill established the following nine criteria for causation (does factor A cause disorder B). Although developed for use in the field of occupational medicine, these criteria can be used in most situations.

1. Strength of the association. How large is the effect? 2. The consistency of the association. Has the same association been observed by

others, in different populations, using a different method? 3. Specificity. Does altering only the cause alter the effect? 4. Temporal relationship. Does the cause precede the effect? 5. Biological gradient. Is there a dose response? 6. Biological plausibility. Does it make sense? 7. Coherence. Does the evidence fit with what is known regarding the natural

history and biology of the outcome? 8. Experimental evidence. Are there any clinical studies supporting the

association? 9. Reasoning by analogy. Is the observed association supported by similar

associations?

The concept of risk and risk factors Risk is the probability that a disease event will occur at a given time or during a given time period The epidemiological triad (agent, host, environment) all contribute risk factors for disease. Risk Factor Identification Using Epidemiological Studies

Cross-sectional study Case-control study Cohort study

http://en.wikipedia.org/wiki/Austin_Bradford_Hill


Standard 2 × 2 table for determining epidemiological associations

Hypothesized risk factor Sick Healthy Row marginal total

Exposed a b a + b

Not exposed c d c+ d

Column Marginal total a + c b + d Grand Total a + b + c + d

Relative risk • is the risk of disease in the exposed group divided by the risk of disease in the non-

exposed group

Relative risk = incidence rate in the exposed group = (a/a+b) Incidence rate in the unexposed group (c/c+d) Interpretation

RR equal to 1: association between exposure and disease unlikely to exist. RR less than 1: decreased risk of disease among those that have been exposed. RR more than 1: increased risk of disease among those that have been exposed.

Sample statement if RR more than 1: A goat housed at Building D was ____ times as likely to develop hemochosis as an animal that was housed somewherre else in the farm.

Odds ratio Odds ratio measures association between exposures and outcomes It is the odds of disease among exposed individuals divided by the odds of disease among unexposed individuals. Odds Ratio = odds of disease among exposed individuals = (a/b) odds of disease among unexposed individuals (c/d) Interpretation- same as RR except that we replace “proportion/probability/rate” with “odds.”

Sample statement if OR more than 1: Being housed at Building D was associated with a ____ fold increase in odds of developing hemochosis.


Outbreak Investigation

What is an outbreak? an “outbreak” is the occurrence of more cases of disease than normally expected within a specific place or group of people (animals) over a given period of time. (CDC)

Outbreak vs epidemic.

Outbreak often preferable; less frightening term. Outbreak is defined by some authors as a limited type of epidemic?

Cluster

an aggregation of cases in a given area over a particular period without regard to whether the number of cases is more than expected.

Steps in Outbreak investigation 1. Evaluate your PREPAREDNESS 2. Verify the OUTBREAK report. 3. Confirm the DIAGNOSIS 4. Formulate a CASE definition and find matching cases. 5. DESCRIBE the disease according to ATP 6. Generate HYPOTHESIS about the disease. 7. Perform an ANALYTICAL study to validate the conclusion 8. Do CONTACT tracing 9. COMMUNICATE the findings. 10. Implement CONTROL measures.

Case definition It is a standard set of criteria for deciding whether an individual/animal should be classified as having the health condition of interest

Case classifications

1. Confirmed case: A case that is confirmed by laboratory assessment 2. Probable case: A case showing typical clinical features of the disease without

laboratory confirmation. 3. Suspect case: A case with has fewer of the typical clinical features.

Line Listing A line listing is a tabular presentation of selected critical items from the collected case reports.

Contact tracing Trace-back. Identification of animals brought into the infected farm in the period

immediately before the disease was first noticed. How and when did the farm get infected? and from where?

Trace forward : Identification of animals that have left known infected farms during the critical period when they may have been in contact with infected animals.


Surveillance and Monitoring

Main purpose: Early detection of disease

Definition- surveillance Adopted from FAO Manual on Livestock Disease Surveillance and Information Systems: “All regular activities aimed at ascertaining the health status of a given population with the aim of early detection and control of animal diseases of importance to national economies, food security and trade”-

Definition- monitoring “Monitoring, on the other hand is a more specific activity/ies that will follow as part of an early reaction should surveillance activities indicate introduction of disease. It will focus more specifically on the identified disease in order to ascertain changes in prevalence level, rate and direction of spread.

Characteristics of Surveillance systems 1. Continuous- Able to establish disease or productivity patterns; Operational even in

the absence of the head of veterinary services; Supported by existing resources, manpower, stakeholders, administrative orders and legislations

2. Systematic- It is guided by accepted standards (qualitative and quantitative); The flow of information follows a stable cycle or direction

3. Relevant- Relevant to national or global concerns, public health concerns, livestock industry concerns

List A diseases are “Transmissible diseases which have the potential for very serious and rapid spread, irrespective of national borders, which are of serious socio-economic or public health consequence and which are of major importance in the international trade of animals and animal products”. Examples: FMD, Vesicular stomatitis; Swine vesicular disease, Rinderpest, Peste des petits ruminants, Contagious bovine pleuropneumonia, Lumpy skin disease, Rift Valley fever

LIST B diseases are “Transmissible diseases which are considered to be of socio-economic and/or public health importance within countries and which are significant in the international trade of animals and animal products.” This group includes such diseases as Rabies, Heartwater, Tuberculosis, New and Old World Screw worm, Brucellosis, and many others.

General Categories of Surveillance Systems 1. Passive- Provider-initiated. Example- the livestock owners submitting reports to

veterinary authorities to seek technical advice on farm animal problems. 2. Active Surveillance System- Health Department-initiated. Example- the Office of the

Provincial Veterinarian conducting a study to detect the presence/absence of bird flu in his area of jurisdiction.


Scope of Surveillance Systems 1. General surveillance- this system allows collection of data about several diseases.

Veterinary authorities require this kind of system. Multiple diseases must be monitored.

2. Targeted surveillance- This system focuses on a specific disease of interest. This system can be observed among farm owners.

Types of Data utilized by surveillance systems 1. Recorded Diagnoses- 2. Clinical signs or Syndromes- 3. Survey Reports to demonstrate freedom from disease. 4. Indirect indicators.

Types of surveillance systems in public and animal health 1. Farmer or Community-Based Surveillance 2. Facility-Based Routine Surveillance 3. Sentinel Surveillance 4. Syndromic Surveillance 5. Special Surveys

Other terms to remember 1. TADInfo- Transboundary Animal Disease Information system 2. One Health Approach – a collaborative, international, cross-sectoral,

multidisciplinary mechanism to address threats and reduce risks of detrimental infectious diseases at the animal- human-ecosystem interface.

3. Phil-AHIS- Philippine Animal Health Information System

Categories of Data According to sources

Primary Data

referring to those data obtained first hand by the epidemiologist to answer the defined objectives of his investigation.

Secondary Data-

include all those existing data which have been collected, submitted, compiled and sometimes published by other persons for purposes which may be different from the study design of the researcher.


Features of methods of data collection Cameron (1999) summarized the features of the four methods of data collection:

Method Expense Speed Up to date Accuracy

Existing data Very cheap Very fast Often out of date

Not good

Interview data Moderately expensive

Fast Usually up to date

OK

Examining animals

More expensive

Slower Very up to date

Good

Collecting specimens

Most expensive

Slowest Very up to date

Best


Disease Control

Definitions • Disease control- reduction of the prevalence of a disease to a level where it is no

longer considered a major health and/or economic problem. • Disease Control Program- larger scale projects to control (or even eradicate, since

control is taken here in its general sense) a disease • Eradication- total elimination of a disease due to removal of its cause • Region- a geographical area equivalent to several neighboring countries. • Zone- an area within a particular country

Identifying priority Diseases 1. The Public Health Threat/Significance

a. What zoonotic diseases occur more frequently? b. What zoonotic diseases cause severe sickness in humans? c. Present mortality and morbidity rates

2. The threat to the livestock industry a. How easily is the disease transmitted from farm to farm? b. What are the losses due to morbidity? c. What are the losses due to mortality? d. What are the effects on trade and commerce?

3. Quantifying disease risk- A scale can be used to quantify the threat from a given animal disease. These are rough estimates.

Components of Risk Analysis (source: FAO MANUAL ON THE PREPARATION OF NATIONAL ANIMAL DISEASE EMERGENCY PREPAREDNESS PLANS)

Risk assessment

Identifying and describing the risks of an event occurring or of taking a particular course of action. Risks can be assessed in a quantified, semi-quantified or qualitative way. The risks can be described as “extreme”, “high”, “medium” or “low”, or by a simple scoring system, for example, 1-5 for the level of risk and 1-5 for the level of potential consequences.

Risk management

This is the process of identifying, documenting and implementing measures to reduce risks and their consequences. Risks can never be completely eliminated. The aim is to adopt procedures to reduce the level of risk to an acceptable level.

Risk communication

This is the process of exchange of information and opinions on risk between risk analysts and stakeholders.


The cost of disease and its control 1. Cost of disease

a. Microeconomic costs: o direct - Costs due to mortality; costs due to morbidity; Costs associated with

treatment (veterinary expenses, drugs); Etc. o indirect costs- Extra labor to nurse the sick animals; Prohibition of sales of

animal products (ex. milk); Increase in food conversion index; Etc. b. Macroeconomic costs- Ex. Impact on trade

2. Cost of disease control a. Microeconomic costs:

o Variable costs- proportional to the number of animals; Diagnostic tests; Vaccines; Drugs; Travel costs; etc.

o Fixed costs- unaffected by the number of animals; Staff costs; Vehicles; Equipment; etc.

b. Macroeconomic costs- Ex. Genetic loss due to the slaughter policy 3. Benefits of disease control

Losses due to the disease without control project - Losses due to the disease with control project Benefit

Strategies against exotic diseases

1. Centralized surveillance system 2. International border controls to prevent the entry of animals, animal products and

other potentially disease-causing products. 3. Regulation of importations 4. Inspection of persons and products arriving at airports and seaports 5. Proper disposal of food wastes from international aircraft and ship.

Strategies against existing local animal diseases 1. Quantitative and qualitative inventory of animal diseases 2. Farm quarantine 3. Disinfection of all personnel and equipment. 4. Appropriate management for contaminated materials 5. Proper controls on swill feeding 6. Implementation of livestock movement controls around the immediate quarantined

or infected area. 7. Construction of physical barriers 8. Vaccination- Selective or Total 9. Removal of animals from areas with high insect population 10. Control of vector population 11. Contact tracing 12. Zoning- The designation of geographic areas in which specific disease control

strategies is to be carried out. These identified areas are the concentric “circles”


around the notorious sites of the animal disease under investigation/control. Each zone follows a different disease control strategy selected.

13. Removal of infected and potentially infected animals 14. Mixed strategies.

Disease control zones (modified from Chapter 5 Early reaction contingency planning—principles and strategies. FAO MANUAL ON THE PREPARATION OF NATIONAL ANIMAL DISEASE EMERGENCY PREPAREDNESS PLANS)

1. Infected premises. It is here that the disease has actually been detected and includes all areas where there are susceptible animals that could have become infected through contact with the diseased animals.

2. Dangerous contact premises. These are premises where overt disease has not yet appeared, but for which epidemiological investigations indicate that there is a high likelihood that infection has been introduced.

3. Infected zone. This is the area immediately surrounding infected premises. While its size and shape are influenced by topographical features, physical barriers, administrative borders and epidemiological considerations, OIE recommends that it should be at least a 10-km radius around a disease center in areas with intense livestock raising and 50 km in areas where extensive livestock raising is practiced.

4. Surveillance (or control zone). This zone is much larger and surrounds one or more infected zones. It may cover a whole province or administrative region (or clan or tribal area).

Documents

Veterinary Epidemiology DrDomingo